1. Field of the Invention (Technical Field)
The invention relates to ultrafine cementitious grouts, particularly ultrafine cementitious grouts suitable for sealing microfractures in rock.
2. Background Art
Grouts are widely used in the construction and mining industries. The most commonly encountered grout is the thin mortar used to fill cracks and crevices in masonry. Grouts used in common residential and commercial construction consist generally of mortars of sand and cement. Where finer grouting is required, such as in decorative or waterproof tile installation, grouts composed of finer sands and plasters are utilized.
Grouts are used in the mining industry to seal fractured rock. The grouts utilized in ordinary mining are similar to those used in masonry construction, and sometimes include sand. As mining becomes more sophisticated, and as artificial subterranean repositories become increasingly attractive alternatives to aboveground storage of nuclear and other hazardous wastes, the importance of sealing microfractures (cracks whose aperture is 100 microns (100 .mu.m) or less) has increased. Extremely small fissures and cracks in repository walls must be reliably closed against the transmission or migration of hazardous fluids and gases.
A principal obstacle to reliable sealing of microfractures with cementitious grouts is the production of grouts with sufficiently small particles. Conventional understanding is that the successful permeation of a fracture with a particle-containing grout requires that the maximum particle dimension should not exceed one-third of the fracture aperture. As a result, ordinary cementitious grouts known in the art contain unacceptably large particle diameters to effectively enter and seal microfractures.
The need to seal microfractures and silty (fine grained) soil, has lead to the use of chemical grouts, such as polyacrylamide and polyacrylate, which are injected as liquids containing no particles. Because they are low-viscosity liquids when applied, chemical grouts adequately penetrate and seal microfractures. The chemical or so-called solution grouts, however, have become undesirable from environmental and worker safety standpoints. Polyacrylamides are known neurotoxins and suspected carcinogens. Polyacrylates can cause serious burns to the skin and mucous membranes. The use of polyacrylamides has been banned, and many manufacturers are phasing out the production of polyacrylates. Moreover, the degradation testing required to demonstrate the longevity of chemical grouts is time-consuming and imprecise; long-term reliability, however, is essential for grout sealing hazardous or nuclear waste repositories.
Interest has recently increased in the US in the use of cementitious grouts to seal microfractures. The most important parameter for evaluating the quality and effectiveness of cementitious grouts is particle size. Portland cements with 90% of the particles smaller than about 10 .mu.ms, and slag microfines with 90% of the particles smaller than 8.4 .mu.ms, have been developed, with the slag composed of the fine grained "dust" which accumulates on precipitation units in the exhaust stacks of steel mills. Some commercially available microfine grouts, however, have mixing instructions which specify large quantities of water (high water to cementitious materials ratios). The amount of water required in these microfine cementitious grout mixes causes undesirable results such as bleed (water separating from the grout as it hardens), reduced unconfined uniaxial compressive strength, and increased permeability.
U.S. Pat. No. 5,346,012 to Heathman and Crook for Fine Particle Size Cement Compositions and Methods describes a microfine grout comprising fine grained Portland cement and fly ash, both with a maximum particle size of about 10 microns, mixed to a water to cementitious material (W/CM) ratio of about 0.7 to about 2.8, preferably 0.9 to 1.3. The fly ash was chiefly SiO2 (54%) and Al2O3 (30%) with minor amounts of other minerals. Recommended additives include superplasitcizer (up to about 3%), and fluid loss control additive (up to about 3%). This reference suggests that the relatively high water content of its preferred mixture can produce grout compositions of low density and low viscosity without unsatisfactory loss in strength, which Applicants' studies refute. No mention is made of the permeability or bleed characteristics of the grout mixture or how it can be prepared.
Unfortunately, neither the slag based grouts or the fly ash based grouts have exhibited the performance necessary for many applications. These performance requirements include the ability to seal fractures in the 10 .mu.m size range, essentially zero bleed, essentially zero porosity and permeability, controlled set time, and reasonable cost.
The grouts that use slag as the pozzolanic component exhibit less compressive strength, have relatively large particle size, and some require additional lime additives. The grout disclosed in U.S. Pat. No. 5,346,012 also has relatively large particle size (no greater than 30 microns as specified in the examples), and requires relatively high water content to attain injectability which result in lower compressive strength, water `bleed` during crystallization that leaves channels of interconnected porosity (`wormholes`), and an undesirable lower density. Although it is clear that grouts with smaller particle sizes are better able to seal the smallest microfractures, no reference provides guidance as to which constituents may be successfully employed in the ultrafine particle size regime (less than about 5 .mu.m), in what ratios one to another, optimal W/CM ratios, or even how the ultrafine grout mixtures can be manufactured.
Consequently, an unmet need remains for a microfine, cementitious grout for use in sealing microfractures, which is easily worked when wet and results in a grout seal having high strength, low permeability and zero bleed.